Move `racket/sandbox` etc to `sandbox-lib` package.
Move `errortrace` collection and documentation to new packages.
Move srfi/4 out of the core.
Move various data structures to `data-lib` pkg.
Move `help` collection out of the core.
Move unstable/contract and unstable/options to their own packages.
Move `compatibility`, pconvert, and mutable pairs to new packages.
Move much of `mzlib` to `compatibility-lib` package.
Move `rackunit` to the `rackunit-lib` package.

The `rackunit/log` utility stays in the core, because it's
used by `raco test` and very small.